This family consists of a single polypeptide chain transporter in the ATP-binding cassette (ABC) transporter family, MsbA, which exports lipid A. It may also act in multidrug resistance. Lipid A, a part of lipopolysaccharide, is found in the outer leaflet of the outer membrane of most Gram-negative bacteria. Members of this family are restricted to the Proteobacteria (although lipid A is more broadly distributed) and often are clustered with lipid A biosynthesis genes.